# Protect lithium battery in parallel

This question is somehow the continuation of this question : Batteries and running time (reed the comments for more information)

And my question is, can I protect my lithium battery in parallel like so ?

I got this "trick" from this document : http://www.ti.com/lit/an/slva139/slva139.pdf

simulate this circuit – Schematic created using CircuitLab

The PMOS have a very low Rdson.

• It will protect you from a battery being connected reverse polarity, that's all. It will not prevent any other problems such as voltage disparity or self balancing currents. Commented Feb 28, 2017 at 16:59
• @JackCreasey And what if I put a diode? Like a zener diode to reduce the loss of voltage? It will not protect them against voltage disparity or self balancing currents? Because I read on other forums that this is possible with diodes. (in the schematic tool I can't find a PMOS with the integrated diode) Commented Feb 28, 2017 at 17:05
• All FETs have an intrinsic diode, but it's in parallel with the active FET structure. Placing a series diode would protect, but of course it will drop the voltage by the diode Vf. Commented Feb 28, 2017 at 17:56
• But why the intrinsic diode is not sufficient? Why do I need to put a series diode? Commented Feb 28, 2017 at 21:15
• Because the intrinsic diode is in parallel with the FET. If you have two batteries connected the two FETs are turned on and there is a balancing current path available. The FETs short out the intrinsic diodes. So you have to put diodes in series with the battery to control battery to battery current. Commented Feb 28, 2017 at 21:50

The only really simple solution to have several parallel batteries supplying the same load is to replace your fets with diodes. The diodes can be low drop schottky types, but you still easily lose 10% of the voltage. Note: no charging is possible through those diodes.

The zener diodes have no place here until you have some complex active circuit.

In theory you could have a multi-input switching regulator and a controller that that takes the next pulse from the battery that has most charge still left. You need only one set of inductive and output circuitry, but the circuit that distributes the intake load properly to the batteries needs to be developed. There exists battery balancing controller ICs for serial batteries. They're a must in modern high power battery systems. Unfortunately I do not know how they could be adapted for this.

• Yes my bad I meant schottky, not zener of course.. Commented Feb 28, 2017 at 21:13

Linear Tech has a few "PowerPath" Ideal Diode Current Balancing Controller just for this purpose.

They have a page where they explain the various options here: PowerPath Controllers & Ideal Diodes.

A Schotty diode will work. You will lose 0.01 V per cell and you will have to remove the diode to charge the cell, or use a bypass switch. These diodes are used on solar panels to prevent a short if one panel is shaded.